Former Russian spy Sergei Skripal and his daughter were probably poisoned by a nerve agent at their front door, British police say.
Retired military intelligence officer Mr Skripal, 66, and Yulia, 33, remain critically ill in hospital after being found slumped on a bench in Salisbury city centre on 4 March.
Counter terrorism police said their efforts were focussed around the Skripal's address in the city.
The risk to neighbours living near the Skripal's remained low, police said.
Specialists identified the highest concentration of the nerve agent, to date, at the front door of the Skripal's house, police said in a statement.
The Metropolitan Police said it wanted to reassure the public that searches around the Skripal's neighbourhood were precautionary.
About 250 counter terrorism officers continue to work on the case.
Forensic work at shops and at a cemetery in Salisbury is drawing to a close.
The Maltings shopping centre, the Ashley Wood compound, and the London Road cemetery have been handed back to Wiltshire Police from the national counter terrorism investigation, Deputy Chief Constable Paul Mills said.
Prime Minister Theresa May has said the chemical used in the attack was part of a group of nerve agents developed by Russia known as Novichok.
The prime minister has said there was "no alternative conclusion other than that the Russian state was culpable," in the attack, and she has expelled 23 Russian diplomats from the UK.